1. Field of the Invention
The present invention relates to GaN-based field effect transistor comprising nitride-based compound semiconductor for use in devices for power electronics and devices for high frequency amplification, and manufacturing process thereof.
2. Description of the Related Art
Since wide band gap semiconductors represented by III-V group nitride-based compound semiconductors exhibit high breakdown field, good electron transferring characteristics, and good thermal conductivity, they are very attractive as semiconductor devices for high temperature, high power, or high frequency. For instance, in a field effect transistor (FET) with AlGaN/GaN hetero-structure, two-dimensional electron gas is generated at the boundary due to the spontaneous polarization and the Piezo effect. Since this two-dimensional electron gas has high electron mobility and carrier density, the hetero junction FET (HFET) employing AlGaN/GaN hetero junction structure has low on-resistance and fast switching speed. These characteristics are very suitable for power switching applications.
Typical AlGaN/GaN HFETs are normally-on type devices in which electric current flows through when there are no bias applied to the gate and in which electric current is blocked by applying negative electric voltage at the gate. In contrast, in applications in power switching, due to securing safety in case the device breaks down, normally-off type devices are preferable in which electric current does not run through when bias is not applied to the gate and in which electric current flows through when positive electric voltage is applied to the gate.
Patent Document 1 shows a metal-oxide semiconductor field effect transistor (MOSFET) of MOS structure wherein an electron supply layer comprising AlGaN, etc., is etched off at the gate portion and an insulating layer is formed on the etching surface of the drift layer. As regards this structure, space between the gate and the drain is formed of a hetero junction structure comprising AlGaN/GaN, and since the two-dimensional electron gas that is generated at the hetero junction boundary has high electron mobility, it can avoid increase in on-resistance in respect of the sheet carrier concentration that is needed for maintaining high resisting voltage. That is, it is a structure suitable for achieving high breakdown voltage with low resistance.    [Patent Document 1] WO 03/071607 Publication
However, since the AlGaN/GaN hetero junction structure is influenced by a phenomenon called, “current collapse” in which amount of current changes with the passage of time, there will be adverse effects upon features of the device such as an increase in on-resistance following application of high voltage between the gate and the drain, an increase in on-resistance at the time of forward flow of current, etc.
As regards the cause of the current collapse, the carrier traps between HFET AlGaN layer and the protection film and the carrier traps within HFET channel layer (drift layer) are believed to influence it.